The manufacture of paper pulp stock from wood by defibrating the wood raw material into fibers suitable for paper making can take place either chemically as a pulp cooking process or by subjecting the deciduous material to mechanical defibrating treatment.
There are two basic types of methods for the mechanical defibrating of wood. The oldest method is the grinding process in which wet logs of suitable length are pressed against the coarse surface of a rotating cylindrical grinding stone. Water in proper amount is also directed onto the grinding surface. The water is necessary because the grinding of totally dry wood leads to the production of useless wood flour. This process results in practically all of the wood material being converted into an aqueous fiber slurry, with the exception of splinters and slivers. This type of fiber pulp stock is generally used as raw material for newsprint.
A more modern mechanical pulp manufacturing method makes use of chipped wood, i.e. wood in the form of wet chips, which are defibrated for example between two disc-like grinding elements which rotate relative to each other and which have a suitable surface structure. This method also results in a pulp slurry which after screening is suitable for paper making.
As is well known, wood contains 20-30 percent of lignin which is an aromatic substance having a large molecular size. Lignin resembles glue to some degree and it acts to bond the wood fibers together. In pulp cooking or other chemical reactions on the pulp, the lignin is mainly dissolved. However, in mechanical defibration the lignin remains on the fibers.
Thus, it is a characteristic of mechanical pulp manufacturing methods that all of the constituents of the wood, that is the cellulose fibers and the lignin bonding the fibers together are present in the produced pulp fiber stock. Moreover, these mechanical pulp manufacturing methods produce an appreciable amount of heat during the defibration process because the mechanical energy is converted to heat and this results in the temperature of the pulp rising to a level which is sufficient to make the lignin soft and sticky.
In recent years a new type or grade of mechanical pulp, so called thermomechanical pulp has gained wide acceptance. This pulp is made from chips in a disc type refiner. In this manufacturing process heat is added in addition to that which is generated by the friction on the chips during the defibration process. Due to the increased temperature in the production of thermomechanical pulp the lignin is softened more thoroughly than in the conventional mechanical pulp processes. This means that the fibers are separated from each other quite easily and in comparatively intact condition so that the fibers are long and pliant. As a result, the quality of the thermomechanical pulp, from the standpoint of paper making, is in many respects higher than that of other mechanical pulps. Thus, in the manufacture of newsprint it is possible to produce paper entirely from thermomechanical pulp, whereas in the case of the use of conventional mechanical pulps (e.g. groundwood) for the production of newsprint it is necessary to add a considerable amount, up to 25%, of chemical pulp or cellulose in order to obtain the required strength for the paper.
It is in general necessary to improve the optical and printing properties of pulps in order to be able to use the same for newsprint and other purposes. Mineral fillers are used in particular to improve the printing properties and opacity of the paper. The introduction of the filler material may take place in one of two different ways, either by mixing the filler with the paper stock or by coating the paper web. In the mixing method the filler is added as a suspension into the pulp stock slurry before the stock enters the paper machine. This is accomplished by introducing the filler as an aqueous suspension of 30-40% solids into the mixing chest following the high consistency stock chest. However, the problem with the use of fillers has always been that they are very poorly retained by the fibers. While it is possible to mix the filler material in the pulp stock slurry so that a homogenous suspension of filler particles and fibers with the water is obtained, when the suspension is fed onto the paper machine wire, where it is dewatered and a continuous web formed from the fibers, a considerable portion of the filler material is removed with the water and the remaining portion of the filler material has a tendency to concentrate on only one surface layer of the web, which results in so called one-sidedness. The quantity of filler added to the paper stock may vary, depending upon paper grade, from 2 to 40% of the weight of the paper produced. The most common filler contents are 5-20%. Talc, clay (kaolin), chalk and other equivalent substances are usually used as fillers. In recent times there has been increased use of high quality fillers such as titanium oxide and zinc sulphide pigments for improvement in the opacity of the paper. Generally speaking the fillers are used for improving the opacity and brightness of the paper and for increasing its receptivity to printing ink, as well as for improving the smoothness and finish of the paper.
As indicated above, while the use of fillers is generally well known, the problem has remained, particularly in the production of mechanical pulp, of retention of the filler by the fibers.
U.S. Pat. No. 3,388,037 describes the production of mechanical pulp wherein the pulp is chemically acted upon, for example by sulfite solution and by sodium peroxide or hydrogen peroxide or hypochlorite. Some fillers may be added along with the chemical solutions in which case, in addition to the whitening effect of the chemical solution there is a further chemical reaction between the fibers and the filler material. In addition to the expense of chemical processing this process suffers from the disadvantage of the loss of newsprint yield because the chemical action consumes a portion of the wood, as well as of the lignin or the like which binds the wood fibers together.